FIG. 4 is a cross-sectional view of conventional boundary acoustic wave device 501 described in Patent Document 1. This device includes medium layer 1 made of LiNbO3, medium layer 2 made of SiO2 stacked on medium layer 1, and interdigital transducer (IDT) electrode 3 made of Au formed at interface 501A between medium layers 1 and 2. In boundary acoustic wave device 501, an SH type boundary acoustic wave as a main mode propagates along interface 501A between medium layers 1 and 2.
The boundary acoustic wave propagates and displaces medium layers 1 and 2. In boundary acoustic wave device 501, an area in which medium layer 2 made of SiO2 is displaced is larger than an area in which medium layer 1 made of LiNbO3 is displaced. This structure prevents the boundary acoustic wave from concentrating at interface 501A, and accordingly, prevents the device from exhibiting superior piezoelectric characteristics of LiNbO3 of medium layer 1, i.e., prevents the device from having a large electro-mechanical coupling coefficient.
Patent Document 1: WO 2004/095699